With a global annual production of more than 4 mio. t, acrylic acid is one of the most produced intermediate products of the chemical industry, as acrylic acid and its derivatives are starting substances for numerous industrial-scale products. Particularly important are the so-called superabsorbents, to which acrylic acid is polymerized together with sodium acrylate (the sodium salt of acrylic acid). The same for the most part are employed in hygiene products (e.g. diapers), but are also used for the storage of water in soils. The esters of acrylic acid are converted to polymers, and the resulting plastics are used in paints or as dispersing agents.
Acrylic acid is industrially produced in a two-stage, heterogeneously catalyzed process. Proceeding from propene, acrolein is obtained at about 350° C. on bismuth molybdates with a selectivity and yield greater than 90 wt-%. In the second step, acrolein is oxidized further on mixed Mo/V/W oxides at about 280° C. to obtain acrylic acid. As by-products the total oxidation products CO, CO2 and water are obtained as well as small amounts of acetic acid from the acetaldehyde obtained during the propene oxidation. Therefore, a multistage processing is carried out subsequent to the reaction.
A particular challenge in the purification of the acrylic acid is the polymerization tendency of the acrylic acid, in particular at elevated temperatures. The polymerization of acrylic acid is a strongly exothermal reaction which releases large amounts of heat. Since the heat in turn leads to an acceleration of the polymerization, acrylic acid tends to explosion-like polymerizations. A polymerization of acrylic acid often leads to clogging and thus to undesired shut-down periods of the entire plant. In this connection, the acetic acid obtained as by-product is to be classified as particularly problematic, as the same promotes the polymerization of acrylic acid as a proton donor.
DE 102 51 328 B4 describes a purification for acrylic acid in which the acetic acid is separated such that it can be obtained from the process in an economically exploitable manner. For this purpose, the reaction products from a mixture of cyclohexane and propyl acetate are absorbed in an extraction column and supplied to a solvent separation column. Cyclohexane is separated therein as top stream and after purification recirculated into the extraction column. The product stream in the bottom of the solvent separation column contains propyl acetate, acetic acid and acrylic acid. This stream is supplied to an acrylic acid separation tower from which acrylic acid is discharged as relatively pure product. The remaining mixture of acetic acid and propyl acetate, which also contains fractions of acrylic acid, is separated in an acetic acid separation tower, where acetic acid is discharged as product and propyl acetate is supplied into the extraction column as solvent.
It was found out, however, that with this purification variant the undesired polymer formation and the fouling both in the acrylic acid separation tower and in the solvent separation column occur to a considerable extent due to the relatively high concentration of the acetic acid. As a result, cloggings already occur within the plant after few weeks of operating time, which is why an intensive cleaning above all within the distillation columns and the associated heat exchangers becomes necessary with corresponding shut-down periods of the plant.
The acetic acid produced thereby has a comparatively high purity of >90 wt-%, but the relatively low sales revenue from the additional production of the acetic acid hardly or only with difficulty makes up for the expenditures for additional investments (e.g. in distillation columns, tank farms, loading etc.) as well as operative costs (e.g. increased consumption of resources). Furthermore, the increased polymer formation and fouling tendency results in distinct reductions in the availability of the plant as well as in acrylic acid product losses due to polymer formation and in additional costs for the disposal of the wastes which are obtained in the cleaning and removal of polymers.
With the catalysts commonly used today and the available reactor sizes, about 400 kg/h of acetic acid are produced in an acrylic acid plant as side reaction. A sales revenue of about 200,000  (at a price of 500 /t) can be achieved therewith. This revenue however is relativized by the acrylic acid losses connected with the separation of the acetic acid. On an assumption of an acrylic acid price of 1500 /t, the revenue already is used up with losses of 130 kg/h of acrylic acid. Acrylic acid nowadays also achieves far higher market prices, and further costs, already described above, for additional investments as well as disposal costs have not yet been taken into account.
A process for the purification of acrylic acid by avoiding a polymer formation and a fouling within the plant is proposed in U.S. Pat. No. 3,798,264. Initially, reaction products are separated as condensate in a condenser and separated from low boilers in a stripping column. The remaining phase containing acrylic acid is separated from the aqueous phase in an extraction column by an extracting agent, wherein a part of the acetic acid already is removed with the aqueous phase. Subsequently, the extracting agent is separated from the acrylic acid and the acetic acid in a solvent separation column. Finally, the acetic acid is removed from the acrylic acid by distillation in an acrylic acid/acetic acid separation column, wherein the acetic acid obtained is supplied to the condensate, the stripping column or the extraction column. To avoid an acid-related polymerization within the acrylic acid/acetic acid separation column, a part of the acrylic acid is recirculated, whereby the acrylic acid concentration in the separation column is adjusted to values between 10 wt-% and 70 wt-%.
It has been noted, however, that the concentration of the acetic acid of 90 wt-% to 30 wt-% as described in the prior art cannot reliably prevent a polymer formation in the separation column and promotes fouling over an extended period. Furthermore, it is disadvantageous in the known processes that a part of the acrylic acid is recirculated not only within the separation column, but also back to the extractor. As a result, the acrylic acid repeatedly passes through the entire separating section including the distillation columns and thus is exposed to a thermal load for a comparatively long time, which in turn promotes the polymer formation and the fouling and at the same time leads to losses of the valuable product acrylic acid.